Gutter leak at joint - how to fix?

I would try to meticulously clean off the silicone. All of it. Then carefully clean off all the residue with something like brake clean or perhaps lacquer thinner. Try those solvents in a small amount in an inconspicuous area to ensure they don't dissolve the plastic material.

I would then seal with either dicor self leveling or proflex. Work it into any gap you can see with your finger.

Report back on success or failure !